VisionFlorida Atlantic University视觉佛罗里达大西洋大学.ppt

6:Vision,Biological Bases of Behavior,Sensory Systems,The brain detects events in the external environment and directs the contractions of the musclesAfferent neurons carry sensory messages to brainEfferent neurons carry motor messages to the musclesStimulus:any energy capable of exciting a receptorMechanicalChemicalThermalPhotonicSensory energies are measurable(unlike ESP),6.2,Sensory Receptors,Receptors are specialized nerve cells that transduce energy into neural signalsReceptors lack axons,form synapses with dendrites of other sensory neuronsReceptors are“mode”specific“Law of Specific Nerve Energies”:sensory messages are carried on separate channels to different areas of the brain Receptors detect a small range of energy levelsEye:400-700 nMEar:20-20,000 HzTaste buds:specific chemicals,6.3,Visual Systems,The function of a visual system is to detect electromagnetic radiation(EMR)emitted by objectsHumans can detect light with a wavelength between 400-700 nMPerceived color(hue)is related to the wavelength of lightBrightness is related to the intensity of the radiationFunctions of visionDiscriminate figure from background(food or rock?)Detect movement(predator/prey?)Detect color(adaptive value of color vision),6.4,Eye Details,An eye consists of:Aperture(pin hole,pit,or pupil)to admit lightLens that focuses lightPhotoreceptive elements(retina)that transduce the light stimulus,Source:http:/www.nei.nih.gov/nei/vision/vision2.htm,6.5,Retina,Light passes through the pupil and is focused by the lens onto the retina at the back of the eyeThe retina consists of three layers of cellsGanglion cell layerBipolar layerPhotoreceptor layer:receptors in this layer transduce lightThe ganglion cell layer is the outermost layer and the photoreceptor layer is the innermost layerIn order to reach the photoreceptor layer,light actually passes through the outer two layers of the retina,6.6,Rods and Cones,Two types of photoreceptors are located within the retinaRods:120 millionLight sensitive(not color)Found in periphery of retinaLow activation thresholdCones:6 millionAre color sensitiveFound mostly in fovea The outer segments(O.S.)of a rod or a cone contain different photopigments that react to light,Source:http:/insight.med.utah.edu/Webvision/imageswv/rodcoEM.jpeg,6.7,Retinal Circuitry,6.8,Adapted from Dowling,J.E.,and Boycott,B.B.Proceedings of the RoyalSociety of London,B.,1966,166,80-111.,Primary Visual Pathway,Information from each visual field crosses over at the optic chiasm and projects to the opposite side of the primary visual cortex,6.9,Visual Pathways:LGN,Signals from the ganglion cells of the retina are sent to the thalamus via the optic nerve/tractThe dorsal lateral thalamic nucleus(LGN)has 6 layersEach layer receives input from only one eyeThe inner 2 layers contain large cells(magnocellular)The outer 4 layers contain small cells(parvocellular)Koniocellular sublayers are ventral to each of the 6 layersNeurons of the LGN project through the optic radiations to a region of occipital cortex termed primary visual cortex(striate),6.10,Visual Pathways:LGN,6.11,Overview of the Visual Cortex,Human visual areas,6.12,Visual areasoccipital lobes,posterior view,V2,V3,V3a,V4v,MT+,left,right,Organization of V1,80,Upper VM,Lower VM,HM,6.14,Visual Transduction,Photopigments are located in the membrane of the outer segment of rods and conesEach pigment consists of an opsin(a protein)and retinal(a lipid)In the dark,membrane NA+channels are open-glutamate is released which depolarizes the membraneLight splits the opsin and retinal apart-Activates transducin(G protein)-Activates phosphodiesterase-Reduces cGMP-closes NA+channelsThe net effect of light is to hyperpolarize the retinal receptor and reduce the release of glutamate,6.15,Retinal Responses to Light,6.16,Receptive Fields,Microelectrodes can be used to record the firing activity of a single sensory neuronSensory neurons have a background rate of firing(impulses/sec)This rate of firing can increase or decrease in response to a stimulus Receptive Field(RF):Those attributes of a stimulus that will alter the firing rate of sensory cellThe general pattern of the RF can be recorded at each level of a sensory system(e.g.from a peripheral sensory receptor,the thalamus,or the cortex)RF analyses can indicate the manner in which sensory information converges from level to level,6.17,Ganglion Cell Receptive Fields,Ganglion cells in the retinal periphery receive input from many photoreceptors while ganglion cells in the fovea receive input from one photoreceptorThe receptive fields of ganglion cells are circular with a center field and a surround field“ON-Cell”:Cell exhibits a low baseline firing rateLight placed in center ring increases firing rateLight placed on surround decreases firing rate“OFF-Cell”Light placed in center ring reduces firing rateLight placed on surround increases firing rate,ON,OFF,6.18,Color Vision Theories,Trichromatic theory(Thomas Young 1802)argued there are 3 different receptors in the eye,with each sensitive to a single hueAny color could be accounted for by mixing 3 lights in various proportionsOpponent theory(Ewald Hering 1905/65)notes that people perceive opponent colors:yellow/blue and red/greenYellow is a primary color rather than a mixture of red and blue-green lightNegative color afterimages suggest that red and green are complementary colors as are blue and yellow,6.19,Color Vision Systems,6.20,Tritanopia deuteranopia protanopia,Color Vision Systems,Primate retina contains 3 types of photoreceptorsEach cone uses a different opsin which is sensitive to a particular wavelength(blue,red,green),supporting trichromatic theoryAt the ganglion cell level,the system responds in an opponent-process fashionA given cell might increase its firing rate to yellow light in the center,but decrease it to blue light,6.21,Center-Surround:LGN/Retina,6.22,Visual Pathways:Striate Cortex,Striate cortex is organized into 6 layersLayer 4c receives information from the parvocellular and magnocellular layers of the LGNThe visual information is the sent to layers above and below layer 4c for analysisMicroelectrode receptive field studies have sought to identify the features of the external world that activate cells in striate cortexOrientation sensitivity:some cells fire best to a stimulus of a particular orientation and fire less when orientation is shiftedSpatial frequency:cells vary firing rate according to the sine wave frequency of the stimulus,6.23,Orientation Sensitivity,6.24,Best orientation,Spatial Frequency,Visual neurons respond to a sine wave grating:Alternating patches of light and darkLow frequency:large areas of light and darkHigh frequency:fine details,6.25,Modular Organization of Striate Cortex,Striate cortex is organized into modules(2500)Stains for cytochrome oxidase(CO)reveal two”CO blobs”in each moduleCells within each CO blob are sensitive to color and to low frequency informationOutside each blob,neurons respond to orientation,movement,spatial frequency and texture,but not to color informationStriate modules showOcular dominance:cells in each half of the module respond to only one eyeOrientation columns:Cells respond to same orientation,adjacent cells are shifted by 10 degreesAre organized at right angles to the ocular dominance columns,6.26,Modules in Visual Cortex,6.27,Cytochrome Oxidase blobs in V1,Visual System Divisions,Magnocellular systemCells from retina terminate in LGN layers 1,2 and then project to layer 4C of striate cortexCarry info on contrast and movement(color insensitive)System is found in all mammalsParvocellular system Cells from retina terminate in LGN layers 3-6 and then project to layer 4C of striate cortexCarry info on fine detail,and color(red,green)System is found in primatesKoniocellular systemSystem projects from LGN to blobs in striate cortexSystem carries color information(blue)System is found only in primates,6.29,Primary Visual Pathway,6.30,GIRKIN AND MILLERSurv Ophthalmol 45(5)MarchApril 2001,Visual Areas in Lateral and Medial Views of Occipital Cortex,Visual Association Cortex,Visual information is transmitted to extrastriate cortex(termed visual association cortex)via two streamsDorsal stream:“where”an object isReceives mostly magnocellular inputProjects to post.parietal association cortexVentral stream:“what”an object is(analysis of form)Receives an equal mix of magnocellular and parvocellular inputProjects to extrastriate cortex(V2,V3,V4,V5)and to inferior temporal cortex(TEO,TE,STS),6.32,Visual Cortex:what/where,6.33,PET study of where/what dichotomy,6.34,Face cells in STS of macaque monkey,6.35,Agnosia refers to a failure to perceive or identify a stimulus by means of a sensory modalityApperceptive visual agnosia is a failure in higher level perceptionPerson has normal visual acuity,but cannot recognize objects based on their shapeProsopagnosia is a form of apperceptive visual agnosia in which the person cannot recognize a face visually,but can do when hearing their voiceAssociative visual agnosia refers to a disconnection between perceptions and verbal systemsPerson cannot name what they seeBalints syndrome relates to failures in spatial/location perceptionOptic ataxia,ocular apraxia,simultanagnosia,Agnosia,6.36,Agnosia different from memory loss,6.37,Apperceptive/Associative Visual Agnosia,6.38,Summary of Visual Cortex,V4:responds to color(and form perception)Lesions of V4 impair color perceptionV5:responds to movementTEO:involved in color discrimination,2-d pattern discriminationTEO projects to area TETE:neurons here respond to 3-d objects(a face or a hand),6.39,